1. Technical Field
The present invention relates generally to internal combustion engines and, more particularly, to a misfire detection apparatus and method with digital filtering to reduce noise-related effects and improve signal to noise ratio for detecting misfire of individual cylinders in an internal combustion engine.
2. Discussion
Pollution control legislation has required motor vehicle manufacturers to reduce exhaust emissions of carbon monoxide, hydrocarbons, and oxides of nitrogen from motor vehicles. To comply with the legislative requirements, most motor vehicle manufacturers generally use catalytic converters on production vehicles to control such exhaust emissions. A catalytic converter typically includes a catalyst and operates at a very high temperature in order to burn unwanted byproducts of combustion so as to reduce hydrocarbons, carbon monoxide and nitrous oxide. However, increased amounts of unburned byproducts that often remain after the occurrence of an engine misfire can lead to damage of the catalytic converter.
More recently, regulatory agencies have proposed that passenger, light-duty and medium-duty motor vehicles with feedback fuel control systems be equipped with a malfunction indicator function that will inform the motor vehicle operator of any malfunction of an emission-related component that interfaces with an on-board computer of the motor vehicle. It is also proposed or required that an on-board diagnostic system identify the likely area of malfunction. Proposals or requirements have set forth catalyst, misfire, evaporative purge system, secondary air system, air conditioning system refrigerant, fuel system, oxygen sensor, exhaust gas recirculation, and comprehensive component monitoring requirements.
As previously mentioned, misfire of internal combustion engines can cause damage to the catalyst of a catalytic converter. With respect to misfire, the identification of the specific cylinder experiencing misfire may be required. Some regulations provide that the motor vehicle manufacturer specify a percentage of misfires out of the total number of firing events necessary for determining malfunction for: (1) the percent misfire evaluated in a fixed number of revolution increments for each engine speed and load condition which would result in catalyst damage; (2) the percent misfire evaluated in a certain number of revolution increments which would cause a durability demonstration motor vehicle to fail a Federal Test Procedure (FTP) by more than 150% of the applicable standard if the degree of misfire were present from the beginning of the test; and (3) the degree of misfire evaluated in a certain number of revolution increments which would cause a durability demonstration motor vehicle to fail an inspection and maintenance (IM) program tailpipe exhaust emission test.
Current and proposed future regulations are also requiring that motor vehicle manufacturers be able to provide information detailing the occurrence of misfires such as identifying which cylinders are responsible for one or more misfires. The misfire information need be collected and stored in memory so that the stored information may be downloaded at a service center and used to determine the cause of misfires in the vehicle. One misfire detection and identification approach is disclosed in issued U.S. Pat. No. 5,361,629 issued Nov. 8, 1994 and titled "Single Sensor Misfire Detection Apparatus and Method for an Internal Combustion Engine". The above-cited patent is hereby incorporated by reference. The misfire detection approach in the above-cited patent senses rotation of a crankshaft and calculates a crankshaft velocity based on the sensed rotation. The calculated crankshaft velocity changes or a compensated velocity change is compared to a predetermined crankshaft velocity range to determine if misfire occurred.
More recent misfire detection approaches are disclosed in pending U.S. patent application Ser. No. 08/468,117, filed Jun. 6, 1995, entitled "Engine Misfire Detection with Compensation for Normal Acceleration of Crankshaft", U.S. patent application Ser. No. 08/469,040, filed Jun. 6, 1995, entitled "Engine Misfire Detection with Rough Road Inhibit", and U.S. patent application Ser. No. 08/468,113, filed Jun. 6, 1995, entitled "Engine Misfire Detection with Cascade Filter Configuration". The aforementioned applications are commonly assigned to the assignee of the present application and are hereby incorporated by reference. The above referenced approaches generally teach misfire detection which includes sorting a plurality of changes in angular velocity over a predetermined series of cylinder firings and averaging the two middle-most angular velocity changes to provide an average change in velocity value. A deviation is determined between the change in angular velocity for a selected cylinder and the average change in velocity value. Misfires are detected as a function of a comparison of the deviation with a threshold value.
A number of conventional approaches are generally susceptible to detecting a false misfire which is really a disturbance caused by a previous misfire. This is often referred to as a misfire induced power train bobble. Additionally, some misfire detection approaches can be tricked into falsely identifying noise, which can be generated by crankshaft torsional vibrations, as a misfire. For example, high engine speed can cause large torsional vibrations that would signal a false misfire detection. Excessive amounts of noise can cause difficulties in detecting occurrences of actual misfires.
In particular, the above-incorporated U.S. application Ser. No. 08/469,040 entitled "Engine Misfire Detection with Rough Road Inhibit" inhibits misfire detection upon detecting noise related events such as a rough road condition. Using this inhibit technique results in a period of time over which misfire detection is deactivated. The inhibit time periods can become quite extensive, at certain operating conditions, especially at low engine speeds and high manifold pressures. While such an approach may be effective to avoiding false misfire detection, it may be preferable to realize continuous misfire detection without the occurrence of inhibit time periods. Accordingly, it is desirable to provide for a misfire detection apparatus and method which would allow for continuous misfire detection, despite the presence of noise-related events.
It is therefore one object of the present invention to provide for a misfire detection apparatus and method of continuously detecting cylinder misfires in an internal combustion engine which effectively reduces or eliminates noise-related events.
It is another object of the present invention to provide for a misfire detection apparatus and method which removes noise signals and achieves enhanced signal to noise ratio to better distinguish actual misfires from noise-related events.
It is a further object of the present invention to provide for a misfire detection apparatus and method which distinguishes misfire induced power train bobble effects from actual misfires.
It is yet another object of the present invention to provide a misfire detection apparatus and method which distinguishes high frequency noise events such as crankshaft torsional vibrations from actual misfires.